Could diamonds reveal evidence for life… four billion years ago?

A paper in today's issue of Nature presents evidence that hints at life 4.25 …

Studying anything about the ancient earth is extremely difficult. Rocks that formed four billion years ago will long since have been beat up, metamorphosed, or melted. Thus, to study the possibility of life four billion years ago, the authors of a paper in today's issue of Nature turned to minute fractions of carbon included in zircon grains that were in turn included in a rock in Australia a mere three billion years ago.

First, let us look at the age of this material. The included carbon is in the form of diamond and graphite, and may be even older than the zircon grains, but we can be confident that it is at least as old as the zircon. The zircons themselves can be dated using lead isotopes, and they ranged from 3.06Ga to 4.25Ga with a typical error range of 0.014 billion years. So we know we have some old rocks.

Second, what can we learn from these ancient "girl's best friends?" When one looks at the 13C isotope levels in the diamonds, they are substantially lighter than expected, with δ13CPDB values around -31‰. Typical mantle values of δ13CPDB are around -6‰; modern diamonds with values as low as their ancient brethren are usually interpreted as resulting from deep subduction of biogenic surface carbon. However, the research team is careful to point out that low δ13CPDB can also be produced inorganically, and thus their data are not conclusive evidence for ancient life.

Ultimately, all we have learned is that there was a much lighter than expected pool of carbon on the ancient earth. But why? Aside from life, light carbon could have come from meteor impacts, interplanetary dust, or it could be the product a period in which there was minimal tectonic activity. Low tectonic activity could allow light carbon to accumulate as a result of interactions with the atmosphere. Then, when mantle convection started (or restarted), this material would be subducted, and the light diamonds that we see could have formed.

Regardless of the source, this study presents a new data set that future studies can use to corroborate or falsify their own hypothesis.

The study itself is well written, and an interesting read for all of the precautions that have to be taken when looking at ancient rocks. Among other interesting tidbits, they used the microscopic texture of zircons in combination with the δ13CPDB to rule out one possibility, namely that the diamonds are younger than the zircons. Additionally, they were looking at the compressibility of zircon (1.6%) to rule out a more recent transformation of graphite into diamond (a process requiring 44% compression).